nix-shell with additional checkInputs

default.nix

{ pkgs, python3Packages }: with python3Packages;

buildPythonPackage {

  name = "myPackage";
  src = ./.;

  propagatedBuildInputs = [
    some-package
  ];

  buildInputs = [
    some-other-package
  ];

  checkInputs = [
    some-check-input-package
  ];

}

shell.nix

let
  pkgs = import <nixpkgs> {};
  myPackage = import ./default.nix {
    inherit pkgs;
    inherit (pkgs) python3Packages;
  };
in (myPackage.overrideAttrs(oldAttrs: {
  checkInputs = oldAttrs.checkInputs ++ [
    pkgs.some-additional-package
  ];
}))